In this talk, pathological science will be discussed. These are episodes in which members of the scientific community have been self-deluded and lost their objectivity in the pursuit of exciting new phenomena. The symptoms of pathological science include the observation of a very weak effect, the lack of a correlation of the magnitude of the effect with the size of the causative agent and an unwillingness to do or accept a definitive experiment. Contemporary examples to be discussed include polywater, infinite dilution and cold fusion.